In a general interior permanent magnet motor, as many magnet insertion holes as the number of poles are formed in advance in an axial direction through a rotor core formed by laminating and fixing a plurality of magnetic steel sheets punched into a predetermined shape. When the rotor is assembled, permanent magnets are inserted into corresponding magnet insertion holes.
When the electric motor is operated, forces caused by the change in attracting force between the magnet and a tooth leading end and by the acceleration and deceleration act on the magnet, and hence the magnet may move laterally in a circumferential direction inside the magnet insertion hole. A large amount of movement of the magnet inside the magnet insertion hole causes vibration and noise, which further causes wearing, cracking, and chipping of the magnet. Therefore, in the vicinity of the lateral ends of the magnet insertion hole, steps or projections are often formed as lateral stoppers against the movement of the magnet.
As an example, in Patent Literature 1, there is disclosed a method of fixing the permanent magnet. In this method, projections are formed in the vicinity of the lateral ends of the magnet insertion hole, and the permanent magnet is arranged between the pair of projections. The permanent magnet is sandwiched by the pair of projections in such a mode that the elastic force of the pair of projections acts on the permanent magnet. In this manner, the permanent magnet has been fixed.
However, when the steps or projections are formed in the magnet insertion hole, the distance between the edges of the hole is reduced in this part, and thus the magnetic flux passes across the magnet more easily as compared to the case in the periphery thereof. Therefore, there has been a problem in that, when the magnet is exposed to a magnetic field caused by a winding current of a stator, the magnet tends to demagnetize in the vicinity of the steps or projections of the magnet insertion hole.